You need a virus with a 100% mortality.
There are a few of them (Ebolla, Hanta, Rabies perhaps HIV) and a good transmission route - one which relies on you biting/humping someone else to transmit it isn't going to get very far very quickly - which is why we haven't all died of rabies in the last few 100years

In practice any virus is likely to mutate into a less harmfull one before it gets too many people or there will be people who are immune.

I very much doubt that Ebola or Rabies would cause the human race to become extinct, as we have, or are near to having, vaccines for them, so if it became a pandemic, as many people would be vaccinated as possible. Also, ebola doesn't have a 100% mortality rate- it's around 60%. It would have to have a 100% mortality rate to ensure no-one could become immune. It would have to be something which had recently mutated, and could keep mutating, and could be rapidly and easily passed between people, probably in the air. It would have to kill people slowly, to ensure that there's enough time for the virus to spread from an infected body before the body died, and infected people would have to be infected with it and able to pass it on before they developed any symptoms and stayed out of people's way. However, I think even in this worst-case scenario, there would still be survivors in the most remote regions of the world.

A short answer to this question would be no, because as the others have noted, it would have to have a 100% death rate.
Transmission must be very effective and the virus must be very robust. What I mean by 'robust' is that it can survive on its own, extracellularly for a prolonged period of time. Remember, a virus is just a chunk of organized proteins and DNA/RNA with maybe a little lipid if it is enveloped.
The human immune system is very capable of recognizing new threats and adapting to them, but at the same time if the pathogen is something that it has not come across before then it would have a hard time countering the invading threat.
I may be wrong here but this could be the reason why everyone was scared of the recent H1N1 swine flu. As I understand it, it had segments of antigens of avian, human and swine flu's. Still, there is no real reason to worry about this media-dubbed threat since the death rate is considerably low in relation to the number of people infected; more people die of the typical seasonal flu by a factor of a few hundred.

Staff: Mentor

It is nice to ask a lot of 'what if' questions. But it also isn't Science. I could say something pointless like 'what if evil Aliens create a perfect virus and spread it thru the atmosphere?'

There is no answer to that - there are no aliens. There also are no viruses that float around in the atmosphere waiting to kill of humans.

Think. Assume there were/are virus diseases like that? What would happen to the virus?
(Hint: virus are very choosy about which host species they infect. Or how would you survive if you eat could nothing but hamburger and all cattle on Earth died. Right now.)

Or.

Bubonic plague killed off huge numbers of European people from the 1300's thru the 1600's. It had fatality rates at nearly 100% in some areas of Europe. We have plague today in the US. And in Europe. It is still around, people are still around too. Why?

You can imagine that our civilization will collapse. If too many people die, perhaps 60% of the population, then the basic infrastructure we have in the Western world that keeps everything going will collapse. Even if you are not ill, your life still depends on the fact that you can get your food from the local supermarket. So, you can expect famines killing billions of people.

Viruses that are so lethal that nobody's immune system can fight it off also have a hard time getting transmitted since nobody with it is walking around spreading it. And, most of the easily transmitted viruses (those transmitted by airborne/respiratory means) move quickly only due to high population densities. Once enough people died off to reduce population density, transmission would slow down, especially since I think we can reasonably presume that if a large portion of the population is dying due to a virus, the remainder of the population is going to voluntarily quarantine themselves to avoid exposure and halt the spread.

I doubt it. With seven billion people and a fair bit of genetic diversity, it's highly unlikely that any virus would be able to wipe out all of us. Perhaps if HIV could spread like the common cold, since HIV has extremely high mortality rates and long incubation periods, but there even seem to be some people immune to AIDS.

As far as influenza pandemics... they've happened before, they'll happen again. It's not going to end human civilization. Even if a massive pandemic killed a billion people (and that's a very generous estimate) civilization would not entirely collapse. We'd be shaken, but in a few years' time it would be business as usual.

With seven billion people and a fair bit of genetic diversity, it's highly unlikely that any virus would be able to wipe out all of us.

Hmmm, I thought that the genetic diversity is extremely small in humans. The seven billion humans there are now illustrate this very well. The population has been growing exponentially starting from a small population. So, the genetic diversity today is the same as it was hundreds of years ago. Also, the genetic diversity of the human populatio was extremely small to begin with. Home Sapiens went through a population bottleneck about 70,000 years ago.

ANd then one needs to consider the fact that the people who live outside Africa are all genetically closely related as they all descent from a handful of people who left Africa about 70,000 years ago.

In a recent documentary on NGC it was mentioed that if you pick a random person in the US and somone from inner Mongolia, they would have less genetic differences than two Chimps living in nearby groups.

Hmmm, I thought that the genetic diversity is extremely small in humans. The seven billion humans there are now illustrate this very well. The population has been growing exponentially starting from a small population. So, the genetic diversity today is the same as it was hundreds of years ago. Also, the genetic diversity of the human populatio was extremely small to begin with. Home Sapiens went through a population bottleneck about 70,000 years ago.

Yes, because there has been absolutely no genetic drift and no new mutations have arisen in any human population in the past 70,000 years. Humans are not a monoculture, we are not genetically identical.

In a recent documentary on NGC it was mentioed that if you pick a random person in the US and somone from inner Mongolia, they would have less genetic differences than two Chimps living in nearby groups.

I'd have too see the number-crunching on that to believe it, and I'm sure you haven't seen the calculations. Until I see the numbers, I really doubt that that would be the case. For one, the US is probably the most genetically diverse human population center in history, so you have to consider whether you're taking an African-American (of various tribal origins back in Africa), an Asian-American (and again, several different countries from which to choose) or a generic European-American mutt like myself. Assuming there is any gene transfer at all between nearby chimp groups, they would almost certainly be pretty similar. Even if individual chimp groups are reproductively isolated, such isolation doesn't seem likely to have gone back farther than it has among different human populations.

Anyways, the point was that we have enough genetic diversity that one virus won't wipe us all out. Looking through history, this seems to be accurate: for any disease that's ever come up, there's been a few people who were resistant to it. Worrying about a virus that will wipe out all humans is nearly as foolish as looking for an antibiotic that will wipe out all bacteria.